The present invention relates to a tube for irradiation equipment for limiting an emergent beam.
The term irradiation equipment is in particular applied, in the text which follows, to particle accelerators, electron accelerators, gamma irradiation equipment and X-ray equipment. For therapeutic and diagnostic use it is essential to limit the cross-section of the beam emerging from the irradiation equipment to the requisite minimum and to screen off the irradiation effectively, at the sides, between the outlet aperture of the irradiation equipment and the surface of the article to be irradiated. At the same time, when using an electron accelerator the proportion of the secondary X-rays and scattered electrons produced by the high speed electrons in the diaphragm system, and manifesting itself outside the actual field of irradiation, should be kept as low as possible in order to reduce the side-effect of the irradiation.
As a further condition, it should be possible to choose as many variations of field dimensions as possible within the range of field sizes imposed by the irradiation equipment. For this reason a variety of devices have also already been disclosed and are used with which the cross-section of the irradiation can be limited and the beam can be screened off laterally.
A first known device consists of a set of diaphragms, of which each is firmly fixed to a tube. The diaphragm and tube can be fixed detachably to the casing of the electron accelerator. For example, the diaphragms are in the form of thick lead sheets which have a central aperture which limits the cross-section of the emergent electron beam. The tube serves to limit the electron beam and screen it off laterally. This relatively simple device suffers from several disadvantages. It is only possible to adjust the beam to fixedly predetermined cross-sections, and the exchangeable diaphragms with attached tube require a great deal of storage space, are expensive, are difficult to handle because of their weight, and must, whenever a change in beam cross-section is necessary, be exchanged, thereby consuming a relatively large amount of time.
Another known device comprises a baseplate on which several diaphragms are located in succession in the direction of the beam. Each of these diaphragms has several diaphragm blocks continuously displaceable at right angles to the direction of the beam. This device permits continuous variation of the diaphragm cross-section and hence of the effective electron beam and does not require an additional tube. A disadvantage of this device is that because of the diaphragms being arranged in succession, a part of the fast electrons is screened off near the object to be irradiated, as a result of which a relatively high proportion of gamma radiation and secondary electrons is liberated, which must be avoided particularly in the case of radiation therapy, and that if the diaphragm aperture is small the diaphragm blocks project beyond the cross-section of the useful electron beam, which makes it difficult, or even impossible, to introduce the device into recesses and, for example, to irradiate the armpits or the throat.
Further, a device is already known which possesses a variable diaphragm built into the accelerator casing, and corresponding tubes of different cross-section which can be attached to the accelerator housing. In this device, the advantages of the variable diaphragm can only be utilized partially because the diaphragm must necessarily be set to the dimensions of the available tubes. Furthermore, this device suffers from all disadvantages already mentioned above for exchangeable tubes.